Effect of nitrogen fertilization on the protein quality of timothy grass and silage

Timothy grass given N fertilizer at the rates of 40, 80 and 120 kg N/ha was preserved in 3 glass-fibre silos of 0.4 m 3. The crude protein content of DM in the grass increased with the increase of N fertilization as follows: N* 14.8 %, N go 18.4 % and N l2o 22.1 %, but the proportion of true protein in crude protein decreased: N* 82 °/o, Ngo 78 % and Nl2O 76 %. The proportion of watersoluble Nin the total Nin the grass was: N4O 27 %, Ngo 30 % and N,20 33 %. The higher was the N fertilization level, the more rapidly was the protein of the grass degraded in the rumen. The amino acid profile of the protein was similar at all the N fertilization levels. The quality of all the silages was good. The NH3 -N fraction of total N was 2.8—3.9 % and the proportion of water-soluble N in total N was 51—55 %, In silage the decrease during ensiling in the proportion of true protein in crude protein and the increase in the proportion of water-soluble N in total N were smaller than in the other silages. The rumen degradability of protein during the first two hours was also lowest in this silage.


Introduction
The protein yields of swards can be increased either by using leguminous plants, e.g.red clover, or by increasing the nitrogen fertilization level of the grass.Little atten- tion has been paid, however, to the quality and the feeding value of the protein in the herbage, when these methods are used.The purpose of this experiment was to study the effect of different nitrogen fertilization levels on the quality and the feeding value of the protein of timothy grass and the changes in its crude protein fraction during ensiling.Corresponding experiments concerning red clover have already been described (Syrjälä-Qvist et al. 1984).

Experimental procedures
The timothy grass used for this experiment was from second-year sward.Basic nitrogen fertilizer was applied in spring at the rate of Index words: N fertilization, timothy, protein quality, silage 100 kg N/ha.The first harvest was taken on 18 June and was used for the experiment described earlier (Syrjälä-Qvist et ai.1984).
After the first cutting, the timothy sward was divided into three parts, which received nitrogen fertilizer at three different levels: 40 kg, 80 kg and 120 kg N/ha.The grass used for this experiment was from the second cut- ting.The harvesting date was 26 July, when the timothy was at the so-called silage stage, or when the ears of the timothy were near emergence or just formed.
Each silage, silage 1 = silage 2 = N go and silage 3 = N l2O , was preserved in a glass-fibre silo of 0.4 m 3. The grass was chopped and AIV II solution (80 % formic acid and 2 °7o orthophosphoric acid) was used as preservative, being applied as the silos were filled, at the rate of 5 1/1000 kg fresh grass.The sampling and analyses were as described earlier (Syrjälä-Qvist et ai.

Silage raw materials
The dry matter and crude protein yields of the timothy at the different levels of nitrogen fertilizer application were as follows:  The increase in nitrogen fertilization clear- ly increased the crude protein content of dry matter, the values being 14.8, 18.4 and 22.1 %, respectively (Table 1).Although this increase was evident in the protein yields, the corresponding dry matter yields did not change very much.
The content of dry matter and water-solu- ble carbohydrates (sugars) decreased with the increase of nitrogen fertilization (Tables I -2), but the content of the other chemical constitutents was fairly similar in the differ- ent grasses.Corresponding results have been obtained in some other experiments (see e.g.McDonald 1981, Salo andSormunen 1976).
In the other criteria describing the quality there were some differences between the grasses (Table 2, Figures I-2).Although the proportion of true protein in crude protein was relatively high in all the samples, it de- creased as the N fertilization level increased, as follows: N 4O 82 %, Ngo 78 % and N l2O 76 %.The proportion of watersoluble nitro- gen in total nitrogen increased with the increase in N fertilization, from 27 to 30 to 33 %.The following percentage values were ob- tained for the degradability of crude protein in the rumen: Although the rumen degradability of crude protein increased with the increase in the N fertilization level (Figure 1), it was quite low in all the samples during the first two hours.An increase in the rumen degradability of grass protein with an increasing N fertilization level has also been noted in another experiment (Möller 1981ref. Setälä 1983).
The N fertilization did not affect the ami- no acid composition of the crude protein of timothy grass, which was similar to that in grasses in other studies (Andersen 1980, Sa-  lo et ai. 1982).Mo (1977) and Andersen  (1980) observed that nitrogen fertilization had some effect on the free amino acids of grass, especially aspartic and glutamic acid, whose content increased with increasing N fertilization.

Changes during ensiling
The quality of all the silages was good (Table 2).The chemical composition of the dry matter of the silages corresponded well with the composition in the raw material (Table 1).
There were slight differences between the silages in the fermentation of carbohydrates and crude protein.In silage N 4O, the sugar content of the grass decreased by only 0.6 %-units, whereas the corresponding decrease in silages N BO and N l2O was more than 4 %-units.The proportion of water-soluble N in total N was lower in silage N l2o than in the other silages.In some other experiments the proportion of water-soluble N increased as the nitrogen fertilization level increased (Ettala et al. 1974), Poutiainen and Rinne  1976).
The rumen degradability of crude protein was higher in the silages than in the corre- sponding grasses, especially during the first two hours of incubation (Figure 1).These re- sults confirm the earlier findings (Syrjälä- Qvist et al. 1984) that the higher is the pro- portion of true protein in crude protein and the lower the proportion of watersoluble N in total silage N, the slower is the degradation of crude protein in the rumen.The increase of the ammonia-N fraction of total N in silage has also been found to be reflected in increased degradation of protein in the rumen (Setälä 1983).In this experiment the proportion of ammonia-N was rather low and similar in all the silages (3 -4 %), so that its effect on the rumen degradability of protein is difficult to evaluate.N fertilization seemed to increase also the level of degrada- tion of crude protein in silage (Figure 1).The amino acid profile of protein did not change very much during ensiling (Figure 2).The total amount of inorganic substances in timothy increased with the rising level of N fertilization (Table 1).As the N fertiliza- tion level of timothy rose, the content of all the minerals that were analysed increased at least slightly, except that of Ca.As no ef- fluent was formed in the silages, the changes during ensiling in the contents of the dif- ferent minerals were small (Table 3).Ac- cording to Varis (1983), high N fertilization increases the total amounts of K, Ca, Na and Mg in grass, but the changes in the amounts mainly depend on the availability of the minerals in the soil.
The total ensiling losses amounted to the following percentages of the fresh material: silage N4O 8.2 %, N BO 8.8 °/o and N l2O 4.1 Although the dry matter content of the raw materials was no higher than 20-24 %, no effluent was formed.The ensiling losses were caused mainly by surface spoilage, which ac- counted for the following losses of fresh ma- terial: N 4O 7.8 %, N BO 8.5 % and N l2O 3.5 %.The fermentation losses made up a very small proportion of total losses.The losses of the different nutrients mainly paralleled the total ensiling losses of the different silages (Table 4).
The palatability to sheep was good in all the silages, the average daily voluntary intake being 2.6-2.7 kg DM/100 kg live- weight.In the grass the in vitro digestibility of organic matter was little higher at fertili- zation level N 12o than at the other levels.
Conversely, in the silages the in vitro digestibility of organic matter decreased with increasing N fertilization level (Table 1).In an earlier experiment the in vitro digestibility of organic matter decreased slightly with increasing N fertilization level in both silage and its raw material (Salo 1978).

Fig
Fig. I.The rumen degradability of the protein of the raw materials and the silages.

Fig. 2 .
Fig. 2. The amino acid contents of the raw materials and the silages

Table 1 .
The chemical composition and the digestibility of the raw materials and the silages.

Table 2 .
The criteria of the silage quality.
protein Soluble N, % of total N

Table 3 .
The inorganic constituents of the raw materials and the silages.